The present invention relates to an X-ray semiconductor detector suitable for a medical X-ray diagnostic apparatus.
In recent years, in the medical fields, medical data of patients have been formed into a database for quick, proper treatment. The patient often uses a plurality of medical institutes. In this case, the patient may not receive proper treatment without data of other medical institutes. For example, the patient may suffer affective reaction with drugs administered by other medical institutes. The patient must be properly treated in consideration of drugs administered by other medical institutes.
Demands have arisen for a database of X-ray photographing image data and digital X-ray photographing images. A conventional medical X-ray diagnostic apparatus uses a silver chloride film. To digitize an X-ray image formed on the silver chloride film, the image on the developed film must be converted into an electrical signal by the scanner. This is very cumbersome and time-consuming.
Recently, digital image data can be obtained by directly photographing an X-ray image with a CCD camera about one inch. However, when, e.g., lungs are to be photographed, an optical device for focusing light is required to photograph an area about 40 cm.times.40 cm, which makes the apparatus large. Further, a resolution is lowered by a conversion of a optical system.
To eliminate the time-consuming, cumbersome processing, downsize the apparatus and improve a resolution, an X-ray semiconductor detector using an amorphous silicon thin-film transistor (a-Si TFT) is proposed (U.S. Pat. No. 4,689,487). FIG. 1 shows an example of the arrangement of this X-ray semiconductor detector.
In FIGS. 1 and 2, a pixel e.sub.1,1 is made up of an a-Si TFT 9105, a photosensitive film (photoconductor film) 9101, and an storage capacitor 9103. Pixels e are laid out in an array (to be referred to as a TFT array hereinafter) made up of several hundred to thousand pixels on each of line and column sides.
The photosensitive film 9101 receives a bias voltage from a power supply 9109. The a-Si TFT 9105 is connected to a signal read line S1 and a gate line Gl, and turned on/off under the control of a gate electrode driver 9113. The terminal end of the signal read line Si is connected to a signal detection amplifier 9115 via a signal read TFT 9107.
When light is incident, a current flows through the photosensitive film 9101 to storage charges in the storage capacitor 9103. The gate electrode driver 9113 drives the gate line to turn on all TFTs connected to one gate line, and then the stored charges are transferred toward the amplifier 9115 via the signal read line S1. The signal read TFT 9107 inputs the charges to the amplifier 9115 in units of pixels, and the amplifier 9115 converts the charges into dot-sequential signals so as to be displayed on the CRT or the like. The charge amount changes depending on the light quantity incident on pixels, and the output amplitude of the amplifier 9115 changes.
In the detector shown in FIGS. 1 and 2, an output signal from the amplifier 9115 can be directly A/D-converted into a digital image. The pixel area shown in FIGS. 1 and 2 has the same structure as in a TFT-LCD (Thin-Film Transistor Liquid Crystal Display) adopted in a notebook personal computer, and can be easily formed into a low profile, small thickness, large-screen display.
The above description concerns an X-ray semiconductor detector of an indirect conversion type in which an incident X-ray is converted into a visible light by a phosphor or the like, and the converted light is converted into charges by the photoconductor or photosensitive film of each pixel. In addition, there is an X-ray semiconductor detector of a direct conversion type in which an X-ray incident on pixels is directly converted into charges.
The X-ray semiconductor detector of this direct conversion type is different from that of the indirect conversion type in the magnitude of a bias voltage and the method of applying it to the charge conversion film. In indirect conversion, a negative bias of several V is applied to only the photosensitive film, and when light is incident on the photosensitive film, charges are stored in the storage capacitor arranged parallel to the photosensitive film and a capacitance Csi of the photosensitive film itself in each pixel. In this case, the maximum voltage applied to the storage capacitor is the several-V bias applied to the photosensitive film. To the contrary, in direct conversion-type, the X-ray/charge conversion film and storage capacitor are series-connected to each other, and receive a high bias of several kV. When an X-ray is incident on pixels, charges generated in the X-ray/charge conversion film are stored in the storage capacitor. If the quantity of incident X-ray is excessively large, charges stored in the storage capacitor increase to apply a voltage of several kV at maximum to the storage capacitor, causing dielectric breakdown of a TFT formed as a pixel switch or the storage capacitor.
For this reason, direct conversion must adopt any measure to protect the storage capacitor from an excessive voltage. For example, as shown in FIGS. 3 and 4 (Denny L. Lee etc., SPIE, Vol. 2,432, p. 237, 1995), a dielectric layer (insulating layer) is formed on the X-ray/charge conversion film to series-connect three capacitors (dielectric layer Cd, X-ray/charge conversion film Cse, and storage capacitor), and charges generated in the X-ray/charge conversion film are partially stored in the capacitance formed by this dielectric layer to prevent dielectric breakdown of the TFT. As shown in FIG. 5, when an excessive quantity of X-ray is incident on pixels, a necessary amount of generated charges is stored in the storage capacitor, and the remaining charges are removed outside the pixel via a protective diode formed on each pixel, thereby preventing dielectric breakdown of the TFT.
In the example of FIGS. 3 and 4, no electrode layers for discharging charges are inserted between the X-ray/charge conversion film Cse and the dielectric layer Cd. Accordingly, resetting of Cd after receiving an image spends a relatively long time, so no moving picture can be obtained.
In the example of FIG. 5, since capacitors are not formed in series, unlike the example of FIGS. 3 and 4, the reset time of Cd is relatively short, and a fluoroscopy mode can be realized. However, when, e.g., a TFT is used as a protective diode, if the drain electrode on the opposite side to the pixel out of the terminals of the protective diode is set to a potential of 0V, i.e., connected to the electrode of the storage capacitor, the pixel potential (threshold voltage) at which removal of charges from the pixel starts becomes small (0 to 4V), the leakage current becomes large, and the TFT cannot be used as a protective diode. This problem can be solved by supplying a positive potential to the drain electrode. However, signal noise may increase or the yield of the TFT array may decrease depending on the layout of the power supply line (bias line) for applying the voltage.
As the number of TFTs used as protective diodes increases, the number of power supply lines increases, and the yield seriously decreases.
At the same time, since the occupation rate of the TFT and power supply line in the pixel increases, a pixel electrode serving as an effective area for the sensor of one pixel and the capacitance of the pixel are difficult to ensure.
In photographing a patient or the like, the X-ray intensity must be set as low as possible. To ensure a large dynamic range, even a weak signal is preferably detected.
The lower limit of this weak signal is determined by the OFF current of the protective diode, a signal shift by the stray capacitance, noise of the operational amplifier, and the like. Since another noise can be reduced by another appropriate means, a change in pixel potential by the leakage current of the protective diode finally determines the lowest detectable signal level of the weak signal. To prevent the change in pixel potential, the value and variation of the leakage current must be suppressed small.
Particularly when X-ray image of a human body is observed, a weak signal is preferably detected to minimize the influence of the X-ray on the human body. Also, dielectric breakdown of the protective diode by application of the pixel voltage must be prevented.
FIG. 6 shows the whole arrangement of an X-ray diagnostic apparatus equipped with an X-ray semiconductor detector using an a-Si TFT. FIG. 7 is an equivalent circuit diagram showing the X-ray semiconductor detector using an a-Si TFT. An X-ray emitted by an X-ray source 9251 passes through an object 9252 to be examined, and is incident on an X-ray semiconductor detector 9253 having an a-Si TFT array structure. The X-ray semiconductor detector 9253 converts the X-ray quantity having passed through the object 9252 into an analog electrical signal. The obtained analog signal is converted into a time-series digital signal by an A/D converter 9257, and the digital signal is stored in an image memory 9258. The image memory 9258 can store data of one or several images, and sequentially stores data at a specific address by a control signal from a controller 9263. An arithmetic processor 9259 extracts data from the image memory 9258, arithmetically processes the data, and stores the result back in the image memory. The processed data in the image memory 9258 is converted into an analog signal by a D/A converter 9260, and the analog signal is displayed as an X-ray image on a monitor 9261.
In FIG. 7, a pixel e.sub.1,1 is made up of an a-Si TFT 9274, a photosensitive film 9270, and an storage capacitor 9273. Pixels e are laid out in a 2,000.times.2,000 array (to be referred to as a TFT array hereinafter). The photosensitive film 9270 receives a bias voltage from a power supply 9271. The a-Si TFT 9274 is connected to a signal read line Si and a gate line G1, and turned on/off under the control of a gate electrode driver 9277. The terminal end of the signal read line Si is connected to a signal detection amplifier 9276.
When light is incident, a current flows through the photosensitive film 9270 to storage charges in the storage capacitor 9273. The gate electrode driver 9277 drives the gate line to turn on all TFTs connected to one gate line, and then the stored charges are transferred toward the amplifier 9276 via the signal read line S1. The charge amount changes depending on the light quantity incident on pixels, and the output amplitude of the amplifier 9276 changes.
In the detector shown in FIG. 7, an output signal from the amplifier 9276 can be directly A/D-converted into a digital image. The pixel area shown in FIG. 7 has the same structure as in a TFT-LCD (Thin-Film Transistor Liquid Crystal Display) adopted in a notebook personal computer, and can be easily formed into a small thickness, large-screen display.
One pixel is formed from one a-Si TFT in FIG. 7, but the pixel is formed from a plurality of a-Si TFTs in an actual device. In some cases, the a-Si TFT may be formed outside the pixel area. For example, the diode may be formed in the pixel as shown in FIG. 8, or stored charges may be converted into a voltage and output as shown in FIG. 9 (AMI (Amplified MOS Imager).
The X-ray semiconductor detector used in the medical fields is demanded for a high S/N ratio and a wide dynamic range. For this reason, a plurality of a-Si TFTs in the pixel must have the same characteristics. Variations in TFT characteristics, particularly, variations in OFF resistance and Vth degrade the image quality of a detected image. If the OFF resistance varies, the leakage current cannot be minimized and increases noise, i.e., decreases the S/N ratio and dynamic range. Variations in Vth offset the output signal and cause fixed pattern noise.
Owing to changes in Vth over time, correction data must be obtained every photographing in order to attain a high-quality image, resulting in a low working efficiency.
As described above, when a plurality of a-Si TFTs are formed in the pixel or when the AMI structure is employed, a high-quality image having a high S/N ratio and a wide dynamic range cannot be obtained due to variations in characteristics of the a-Si TFTs.
In many cases, variations in TFT characteristics are caused by variations in TFT shapes owing to mask misalignment in manufacturing a TFT array. To make the TFT characteristics uniform, the shapes of TFTs must be formed uniform.
FIGS. 10A, 10B, 10C, and 10D sequentially show the steps in manufacturing a TFT array. In FIGS. 10A, 10B, 10C, and 10D, source and drain electrodes are formed. After an electrode material metal (e.g., Al or Mo) is deposited (1), a resist is applied (2), exposed to light via a mask (3), and etched (4), thereby forming electrodes. The TFT is formed from respective layers (gate electrode, insulating layer, pixel electrode, and the like), and these layers use predetermined masks. To form the TFT into a designed shape, these masks must be accurately aligned. However, since misalignment of the masks is inevitable to a certain degree, the TFT array must be designed in consideration of mask misalignment so as to obtain desired performance even in the worst case.
L. S. Jeromin et al., have introduced a two-dimensional X-ray semiconductor detector in which an amorphous Se layer for converting an X-ray into charges is stacked on an a-Si (amorphous silicon) TFT (Thin-Film Transistor) array (SID 97 DIGEST (1997) p. 91).
FIG. 11 shows a conventional TFT array. This TFT is a top gate electrode type a-Si TFT. An SiO.sub.x film 9302 is formed on a glass substrate 9301. A drain electrode 9313, a source electrode 9303 made of ITO, and a lower capacitor electrode 9305 are formed on the SiO.sub.x film 9302. An a-Si layer 9304, a capacitor insulating film 9306, an insulating film 9307, a gate electrode 9309, a passivation film 9310, an upper capacitor electrode 9308, and a pixel electrode 9311 made of ITO are formed on the resultant structure. The passivation film 9310 is generally made of an SiN.sub.x film. Since SiN.sub.x is poor in step coverage, interlevel short circuits readily occur between the gate electrode 9309 and pixel electrode 9311, resulting in low manufacturing yield. Since SiN.sub.x is formed by a CVD process, it cannot be made thick and is formed to a thickness of about 2,000 to 3,000 .ANG. (angstrom). If this thin SiN.sub.x film is used, an electrostatic capacitance is generated between the gate electrode 9309 and pixel electrode 9311, distorting or delaying a gate electrode signal pulse.
The conventional X-ray semiconductor detector uses ITO for the pixel electrode. A chest photographing detector requires an area of 40 cm.times.40 cm for the pixel area. However, the ITO film is difficult to deposit uniformly in this large area. ITO is patterned by a hydrochloric acid-based etching solution using a photoresist as a mask. Since the etching rate of ITO by the hydrochloric acid-based etching solution changes depending on the crystallinity, the etching rate is high for bad crystallinity and low for good crystallinity. Etching is performed until all the ITO film is removed, namely a portion having the lowest etching rate within the plane is completely etched. At this time, a portion having a high etching rate is excessively etched. Especially, ITO is amorphous-like at the interface between the organic insulating film and ITO.
Since, therefore, the etching rate at the interface is very high, the etching solution penetrates to cause serious side etching. When a 1,500-.ANG. (angstrom) ITO film is etched in an 8 cm.times.8 cm pixel area for an over etching rate of 10%, the side etching amount is 0.5 .mu.m at minimum and 10 .mu.m at maximum. In this case, when the pixel electrode resist pattern size is 100 .mu.m.times.100 .mu.m, the maximum pixel area is 9,900 .mu.m.sup.2, and the minimum pixel area is 6,400 .mu.m.sup.2 which is 64.6% of the maximum pixel area. If the pixel area varies between pixels in this manner, the signal amount varies not to obtain an accurate image.
In manufacturing a TFT array, an electrostatic TFT error may occur and appears as a point or line defect on a detected image to degrade a display image.
FIG. 12 is a sectional view showing the pixel area of the X-ray semiconductor detector. In FIG. 12, a gate electrode 9122, a gate line 9123, a storage capacitor line (including an storage capacitor electrode) 9124 are formed on a glass substrate 9121, and an insulating film 9125 is formed on them. An a-Si film 9126 serving as a channel formation layer for an a-Si TFT is formed on the insulating film 9125, and a silicon nitride film (not shown) is formed as a stopper insulating film on the a-Si film 9126. N.sup.+ -type a-Si films (not shown) serving as source and drain electrodes are formed in regions corresponding to the two sides of the a-Si film 9126, and connected to source and drain electrodes 9128 and 9127. The drain electrode 9127 is connected to a signal read line 9129 formed from the same layer in the same step. An insulating film 9130 is formed on the electrodes 9127 and 9128 and the signal read line 9129. A pixel electrode 9131 is formed on the insulating film 9130 via an opening formed in this insulating film. An X-ray/charge conversion film 9132 is formed on the pixel electrode 9131, and an upper electrode 9133 is formed on the X-ray/charge conversion film 9132.
In the conventional structure, only one insulating film 9130 is interposed between the signal read line 9129, pixel electrode 9131, and X-ray/charge conversion film 9132. Large noise may be generated by capacitive coupling between them. Further, only the insulating film 9125 is interposed between the signal read line 9129, lower storage capacitor line 9124, and the like. Also at this portion, a large noise component may be generated by capacitive coupling between them. The noise component of the signal read line is amplified by a detection amplifier, failing to obtain an accurate photographing result (diagnostic result).
As described above, in the conventional X-ray semiconductor detector, large noise may be generated by capacitive coupling between the signal read line and another conductive region, and an accurate photographing result is difficult to obtain particularly for a weak signal.